Rifaximin is a semi-synthetic, rifamycin antimicrobial drug with in vitro activity against Gram-positive, Gram-negative, and anaerobic bacteria. It acts by inhibiting bacterial ribonucleic acid (RNA) synthesis. Rifaximin is chemically termed as (2S,16Z,18E,20S,21S,22R,23R,24R,25S,26S,27S,28E)-5,6,21,23,25-pentahydroxy-27-methoxy-2,4,11,16,20,22,24,26-octamethyl-2,7-(epoxypentadeca-[1,11,13]-trienimino)benzofuro-[4,5-e]-pyrido-[1,2-a]-benzimidazole-1,15-(2H)-dione, 25-acetate(I).

Rifaximin is used for treatment of travelers' diarrhea caused by noninvasive strains of Escherichia coli. 
WO 2009/137672 discloses a method of treating bowel disease (BD), comprising administering a gastrointestinal cleanser to a subject in need thereof; and administering a therapeutically effective amount of an antibiotic.
Rifaximin was first disclosed in U.S. Pat. No. 4,341,785 which also discloses a process for its preparation and a method for crystallization of rifaximin by using suitable solvents or a mixture of solvents. This patent does not mention polymorphism of rifaximin.
U.S. Pat. No. 4,557,866, and its equivalent CA 1215976, disclose processes for the preparation of rifaximin.
WO 2007/047253 discloses a pharmaceutical composition of hydroxybutenyl cyclodextrins with antifungal azole compounds. However, this application does not provide any enabling methods nor proof of advantages of such a complex.
WO 2008/035109 discloses the amorphous form of rifaximin.
U.S. Pat. No. 7,045,620, discloses various crystalline polymorphic forms of rifaximin which are termed as rifaximin α, rifaximin β, and rifaximin γ. These polymorphic forms are characterized using X-ray powder diffraction. According to U.S. Pat. No. 7,045,620, the presence of water within the crystallization solvent plays an important role in crystal formation. Thus, rifaximin polymorphs undergo changes with a change in the moisture content, and interconversion of one form to another occurs with an increase or decrease in the water content.
U.S. Pat. No. 7,045,620 further discloses rifaximin a which has a water content between 2.0% and 3.0%, rifaximin β which has a water content between 5.0% and 6.0%, and rifaximin γ which is poorly crystalline and has a water content between 1.0% and 2.0%.
EP 1698630 discloses further polymorphic forms of rifaximin termed as 6 and E. The stability of these forms also depends upon the water content.
However, all these forms are hygroscopic and they have a tendency to interconvert from one to another. Thus, these forms are difficult to handle as well as store and they require controlled conditions, specifically, humidity and temperature during handling and storage.
Thus, transformations of polymorphic forms of drug substances are of great disadvantage, because they cause difficulties in fulfilling pharmaceutical requirements and specifications. The physicochemical properties of products that exhibit such polymorphic change vary according to the actual ratio of polymorphic forms. This causes further difficulties while formulating the polymorphic forms into suitable dosage forms.
Also, as rifaximin is sparingly soluble in water, the formulation chemist finds it difficult to prepare a consistent formulation using the known polymorphic forms. Hence, there is a need to prepare rifaximin in a form which is suitable for formulation and has increased solubility and stability.